This invention relates to a reciprocating power tool and drive mechanism for same, such as for a automobile windshield or other glazing removal operations, which involve cutting a urethane bed around the windshield or glass perimeter and separating the glass or windshield from a vehicle.
In the process of securing a glass windshield to a new car or truck, the manufacturer first extrudes a bead or “bed” of urethane onto the “pinch weld” extending around the perimeter of the window opening. The urethane bed bonds to the pinch weld surface. The glass windshield then is seated on the bed and bonds to it, forming a liquid-tight seal. The urethane bed is hard and tough and not easily penetrated. It commonly has a thickness of about ¼ inch and width of about ⅓-¾ inches, widening to about 3½-4″ or more at the corners.
If the windshield is to be replaced, an installer severs the urethane bed along its length, just beneath the inside surface of the glass, leaving the base portion of the bed in place, without causing damage to the glass, the vehicle frame or the pinch weld. The cut surface of the bed is then primed, a new bead of urethane is deposited on the primed surface, and a replacement windshield is seated onto the fresh bead to bond it in place.
Until the late 1980's installers used a manually operated knife to sever the urethane bed. The knife was generally L-shaped, having a narrow sharpened blade and a shank or handle equipped with hand grips. The installer would first work the point of the blade inwardly to penetrate the bed and would then reciprocate the blade with short in and out strokes while pulling the blade sideways to saw through the urethane bed along its length. Commencing about 1990, two types of powered tools were introduced commercially, to replace the manually operated tools. These powered tools mimicked the manually operated knife in several structural and operational respects. The first type of tool involved an L-shaped blade and shank powered by an electric motor. The blade was oscillated from side to side. The blade was pointed at its end and narrow (about ½ inch wide), to facilitate initial manual penetration. The side edge of the blade was sharpened, so that a lateral cutting action was produced. The second type of tool involved reciprocating a blade which would extend in and out of a sleeve. The blade was actuated by an electric motor. Again, the blade was pointed and narrow (about ½ inch width), to facilitate initial penetration. The “throw” or distance advanced by the blade was short, about ½ inch.
In use, these early power tools were characterized by several problems. More particularly:                Cutting with them was slow.        The depth and width of the cut were small, requiring a great many cuts to complete the job.        Typically it would take about 15-20 minutes to cut the windshield free.        Using the prior art assemblies was laborious. The installer would have to apply a sideways pull to these tools, particularly when cutting along the horizontal legs of the bed.        It was common to crack the glass when first penetrating the bed. This could leave broken glass on the dash and elsewhere, which was undesirable.        The cuts made by the tools were shallow. One could only cut into the bed with these tools about ½ inch at a time. This meant that at the corners, where the bed width was greatest, it would take prolonged cutting to complete severing the bed. In practice, the installer would revert to using a hand knife at the corners or would break off the glass at the corners and have to later complete the cut by hand of the corner urethane with its embedded glass.        In the course of hand cutting the corners, installers would commonly press hard with their head against the windshield to assist in cutting, leading to neck injuries.        The cut surface produced by the blades of these powered tools was irregular, tagged and undulating. This was undesirable as the newly extruded bead would in part reproduce this underlying irregularity and the new windshield would then not seat flush against the bed surface. This could lead to a poor seal and subsequent leaking.        The electric motor-driven tools of the prior art were generally bulky and difficult to work with in the confined space of the apex between the vehicle dashboard and the inside surface of the windshield.        The prior art blades were fragile and expensive; breakage was a problem.        It was necessary to use soapy water as a coolant and lubricant for the narrow, rapidly moving blades. After the cut was complete, the cut surface needed to be carefully washed to remove soap, as it was detrimental to getting a good bond when new urethane was laid on to the old bed.        
Against this backdrop, an air powered windshield removal assembly is described in U.S. Pat. No. 6,862,968, issued Mar. 8, 2005 and in U.S. Pat. No. 7,121,007 issued Oct. 17, 2006, both naming inventor Ogston, and both being commonly owned by Crystal Glass Canada Ltd., the owner of the current application. The assembly included a modified reciprocating air gun which attached through a shaft to a unique, bell-shaped blade. The blade was wide at its front end and narrowed toward its rear (shank) end. The shank was connected to the reciprocating shaft of the air gun.
While the above Ogston assembly was a marked improvement over many of the other prior art devices, it presented its own unique issues, including:                The sharp corner edges at the front end of the blade could damage either or both of the pinch weld of the vehicle holding the windshield, or to the dashboard or upholstery of the vehicle.        Due to the extreme flexing requirements for the blade during use against the windshield, including twisting motions, the blade could break at the stress point between the bell shaped side edges of the blade and the straight shank side edges where the blade connected to the shaft. As well, the shaft could break at the stress point at the connection to the blade.        Penetration of the urethane bed, particularly at the corners where the bed is deepest, was sometimes still difficult.        Cutting around the corners of the windshield presented some difficulty, with the sharp corners limiting movement around the corner.        Cutting was primarily in the vertical (i.e., forward) direction, as the front end of the blade cut into the urethane bed. To move the blade along, particularly in the wider urethane bed at the bottom of the windshield, the installer would reposition the blade for each new cut into the urethane bed. While the wider blade made the operation much faster than with the prior art designs, it still took several minutes to remove the windshield.        In some newer vehicle designs the urethane bed has become wider as the windshield have moved to more aerodynamic designs, and the urethane bed has become an integral part of the structural strength of the vehicle. The bell-shaped blades could not be simply elongated to accommodate the needed deeper penetration without the blade becoming too fragile when flexed, i.e., if elongated beyond about 7.75 inches, the bell-shaped blades would break at their stress point (i.e., where the blade side edges joined to the shank).        The air gun, while suitable for some applications, did not meet all customer needs. Cordless and electrical power units were also desirable.        
These blade breakage problems described above were addressed with a new blade design in U.S. Pat. No. 7,908,945, issued Mar. 12, 2011 to Ogston, and owned by Crystal Glass Canada Ltd. However, the reciprocating power units used to drive the blades still have many of the problems outlined above, leading to breakage of the external shaft at its forward end where it connects to the blade and/or breakage at the connection of the external shaft to the internal drive shaft of the power tool. As well, the reciprocating drive mechanism used in the power tool were often a simple adaption of a reciprocating saw drive mechanism, which is not designed for the high impact, short linear stroke and loading for the complete cycle needed to drive a windshield removal tool, leading to tool failure. Many reciprocating drive mechanisms use a wobble mechanism to convert rotary motion to reciprocating linear motion. Others use a scotch yoke type mechanism to convert rotary motion to reciprocating linear motion.